A simple system for isolation of bovine or human retinal blood vessels has been adapted to measurements of I125 insulin binding in vitro. Affinity (Kaff) and dissociation (Kdiss) constants for I125 insulin binding to blood vessels and number of binding sites can be calculated. Using such a system, I125 insulin or I125 growth hormone binding by retinal blood vessels will be determined and: 1) effects thereupon of somatostatin glucagon, epinephrine, cortisol, ACTH, prolactin, and serum of normal and diabetic patients with and without retinopathy; 2) intracellular localization by EM-autoradiography; 3) binding to retinal vessels after intracarotid injection of I125 insulin to living cats; and 4) binding to retinal vessels in normal rats and in rats with streptozotocin diabetes treated and not treated with insulin. Insulin effects on retinal blood vessels (bovine, human) will be tested in vitro by measurements of: A) cAMP, cGMP, phosphodiesterase, adenyl cyclase, guandidyl cyclase; B) enzymes of basement membrane synthesis (glucosyltransferase and glycosyltransferase); and C) aldose reductase. Similar studies will be made to isolate and purify protein binding insulin receptors from capillary cell wall and from cultured pericytes. The information will help clarify the effects of insulin and human growth hormone on the retinal capillaries and pericytes, and provide a model system for studies of drugs on insulin or HGH binding by retinal blood vessels. Studies on human retinal blood vessels will allow evaluation of the effects of diabetes on insulin and growth hormone binding by retinal blood vessels.